It is generally known that a gradual increase in a torque in a rotating direction at the initial stage in a process of engagement of the clutch gives good drive feelings and enables an operator to carry out smooth operation.
An arrangement having a series of elastic cushioning plates connecting a spline hub of a clutch disc with a pair of friction facings is also known to obtain the gradual increase in the torque at the initial stage. However, such cushioning plates must have sufficient rigidity for transmitting torque, so that there is a limitation to obtaining gradual increase in torque at the initial stage by such arrangement.
In order to overcome the above mentioned problem, another arrangement, as shown in FIG. 18, is proposed in the Japanese Examined Utility Model Publication No. 59-2337. In this arrangement, a pressure plate 200 has a rear surface forming thereon with a series of projections 201 for contacting with a diaphragm spring 202. An annular wave wire ring 210, rising and falling in an axial direction, is mounted between the projections 201 and an outer periphery of the diaphragm spring 202 so as to obtain gradual increase in torque at the initial stage.
However, in such the arrangement, as shown in FIGS. 19 and 20, because the wave wire ring 210 is formed in a wire rod, sliding quantity of the end portion 212 or the other end portion 213 of the wave wire ring 210 will be magnified by accumulating sliding quantity of each projection portion 214, 215, 216, 217, 218, 219 and 220 from intermediate projection 217 to the both end portions 212 and 213. For example, as shown in FIGS. 19 and 20, let l be the same sliding quantity of each projection 214, 215, 216, 217, 218, 219 and 220 and let the datum point be the intermediate projection 217, the sliding quantity of the projection 218 next of the datum point is l, a sliding quantity of the projection 219 become 2.times.l, a sliding quantity of the projection 220 become 3.times.l, so that the quantity at the end portion 212 become 4.times.l. Thus, abutting portions of the rear surface of the pressure plate 200 (FIG. 18) and the diaphragm spring 202, with the wave wire ring 210, are partially worn away as near to the end portions from the datum point, so that the disadvantage may occur, that is, the dislocation of fulcrum as to a quantity of partial abrasion.
In order to avoid the disadvantage, another arrangement is proposed in the Japanese Unexamined Patent Publication No. 62-13830. In the arrangement, a wave wire ring is provided with a series of saddles so that the deflected quantity is distributed among the specified places where the saddles are mounted by applying the clutch engage load to the wave wire ring through the saddles. However, in such arrangement, many complicated members, i.e. saddles, must be mounted on the wave wire ring. This causes not only time for manufacturing, but also breakdown due to the coming off of the saddles in use.
Accordingly, it is an object of the present invention to provide an improved clutch cover assembly, overcoming the above mentioned disadvantage, in which a damper member for obtaining a gradual increase in the torque at the initial stage of engagement of a clutch is obtained by a series of bow-shaped wave wires arranged with spaces therebetween in a circumferential direction.
In the prior art arrangement of FIG. 18, the clutch engage load is applied to wave wire 210 by the diaphragm spring 202, while the clutch is being engaged, so that the wave wire 210 may be entirely pressed by the diaphragm spring 202. Therefore, the spring force of the wave ring 210 may be weakened. Thus, the spring constant of wave wire 210 is apt to dislocate.
Moreover, in the prior art arrangement of FIG. 18, because the placing of the fulcrum P10 is determined by balancing the force of the diaphragm spring 202 with the reaction force of the wave wire spring 210, the distance L10, measured from the friction surfaces of cutch facings 211 and the pressure plate 200 to the fulcrum P10, may be unstable and apt to shorten because of decay of the wave wire spring 210 in use thereof.
Accordingly, it is a further object of the present invention to provide an improved clutch cover assembly, overcoming the above mentioned disadvantage, which clutch cover assembly comprising a fulcrum portion pivoting an outer periphery of a diaphragm spring while the clutch is being engaged and a damper member pivots the outer periphery of the diaphragm spring while the clutch is being disengaged.
Another prior art, Japanese Unexamined Patent Publication No. 61-228127 discloses an arrangement in which a wire ring is provided with sleeves of different diameters for two-stage absorbtion. However, in such arrangement, the sleeves must be manufactured and mounted on the wire ring, one, by one and results in high cost. Durability and accuracy, which is intended in the present invention, may not also be obtained.